Impulse generator



April 1944' c. E. HALLMARK 2,345,668

' IMPULSE GENERATOR meg March 14, 1942 FIG! h SYNCHRONIZING PULSE ILGENERATOR FIG. 2

l5 E L 0 TIME INVENTOR E. HALLMARK Patented Apr. 4, 1944 2,345,688IMPULSE GENERATOR once a. Hallmark, Fort Wayne, Ina, mumto FarnsworthTelevision and Radio Corporation a corporation of Delaware ApplicationMarch 14, 1942, Serial No. 434,730

4Claims. (Cl. 250-27) This invention relates generally to scanningcircuits for controlling the deflection of an electron beam in cathoderay tubes such as are used in television, and more particularly tocircuits for generating or developing voltages of saw-tooth wave form ofthe type which are controlled by synchronizing pulses of square top waveform. e It is frequently desirable in television systems to utilizepush-pull modulation circuits particularly in scanning control circuitswherein'it is desired to introduce control voltages for altering thescanning voltage to correct for Keystone distortion in iconoscope tubesor other cathode ray tubes. It is necessary in modulators of this typeto eliminate higher order distortion which occurs as a result ofmodulating a line scanning frequency voltage by the field-scanningfrequency voltage. While it is comparatively simple to eliminate thefundamental field frequency component in the modulated scanning voltage,it is more diiilcult to eliminate the higher order field frequencycomponents. It is possible to improve .the performance of the modulator,and thereby facilitate elimination of distortion by supplying a balancedinput to the push-pull modulator thereby obtaining a more nearlybalanced output in the modulator.

Push-pull modulation is also desirable in scanning circuits designed tosupply a balanced scanning voltage to the deflector plates of a cathoderay tube, because of the fact that various electrodes of such tubes mustbe balanced vide' a circuit for generating separate voltages .ofsaw-tooth wave form,.equal in amplitude but of opposite polarities.

The invention comprises a novel saw-tooth wave generating circuit,including a first timesapacitance element of the second time-constantcircuit is gradually discharged, thus forming a pair of graduallysloping voltage waves, one increasing and the other decreasing.Synchronizing impulses or other recurrent impulses are fed into theinput of the thermionic discharge device and each synchronizing impulseovercomes the normal grid bias therein to render the thermionicdischarge device periodically conductive, whereby the capacitanceelement in'the first discharge circuit is suddenly discharged.

and the capacitance element in the second discharge circuit is suddenlycharged. Thus, there appears across the resistance element in eachtime-constant circuifia train of saw-tooth voltage waves of equalamplitude but of opposite polarity or phase.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawing, and itsscope will be pointed out in the appended claims.

In the accompanying drawing, Fig. '1 is a circuit diagram illustrating-the invention, while Fig. 2 is 'a graph illustrating the wave formsofthe output of the circuit shown in Fig. 1.

Referring to the drawing, Fig. 1 thereof illustrates a circuit forgenerating saw-tooth waves of the type described heretofore. The blockI, labeled synchronizing pulse generator," may be any conventional formof pulse generator such as, for example, oneof the type used intelevision systems designed to generate pulses having square topped waveform, as illustrated in Fig. 1. If it is desired to produce 30 frames.per second and a 525 line picture, the pulse generator I would bedesigned to generate a frequency of the order of 15,750 cyclesper-second.

The output of pulse generator I is coupled through a condenser 2 to thegrid of a thermionic discharge device or vacuum tube 3. A

grid leak resistor I is connected between the rid and cathode of thetube 3. The anode circuit of tube 3 includes a first time-constantcircuit comprising a condenser 6 connected between the anode of tube 3and ground, together with a reconstant circuit consisting of capacitanceand a resistance elements together with a second substantially identicaltime-constant circuit and a source of potentialfor charging thecondensers. The first time-constant circuit is in the plate circuit of athermionic discharge device and the second time-constant circuit is inthe cathode circuit of the same device. The capacitance element of thefirsttime-cohstant circuit becomes gradually charged while at the sametime the sistor I connected to a source of plate supply "labeled 13+.The anode output voltage developed across the resistor I may be appliedthrough conductor III to whatever circuit maybe desired, such as amodulator or amplifier.

A second time-constant circuit is connected between the cathode of tube3 and ground. and

consists of a condenser l2 connected in parallel with a resistor II. Thecathode circuit is also to the same circuit as connected to a conductorl4 whereby the voltage developed across the resistor It may be appliedthat appearing in conductor ll. 7

. tical in wave form but opposite in phase.

In operation, the output of the synchronizing pulse generator whichconsistsmf a series of square top impulses, such as that illustrated inFig. 1 which may, for example, have a frequency of the order of 15,750cycles is impressed on the grid of tube 3 through coupling condenser 2to overcome the normal bias which appears. across grid resistor 5. Thus,tube 3 becomes conductive periodically, according to the frequency ofthe synchronizing pulse.

Referring to Fig. 2 of the drawing, it will. be seen that at zero time,or the initiation of a cycle, the saw-tooth wave voltages el and e2begin to build up across resistors 8 and I3 neglecting the drop in tube3 of Fig. 1. This is true because of the fact that the time-constantcircuits 6, 8, l2 and I3 respectively are connected in series across theanode supply. More particularly, and again referring to Fig. 2, it willbe noted that datum line I5 is drawn to indicate an average orpredetermined level of the saw-tooth wave and this will, therefore, beassumed as the zero voltage of the saw-tooth. If it be assumed thatcondenser 6 has just discharged, it will subsequently charge at agradual rate from the plate supply through resistor 8, and the voltageacross resistor 8 will gradually increase as shown by the slope of el inFig. 2. At the same time the voltage across condenser l2 will begradually decreased as its charge leaks away through resistor l3, andthere.- fore, the voltage across resistor I 3 will have a slope asindicated by e2 in Fig. 2. The condensers 6 and I2 will continue,respectively, to charge and discharge until such time as a synchronizingimpulse from source I changes the bias on the grid of tube 3 to renderit conductive. At this time condenser 6 is rapidly discharged to thevoltage represented by the datum line l5 and condenser I2 is rapidlycharged to the same voltage. The synchronizing pulse from source I beingof short duration, tube 3 becomes immediately non-conductive so that thecharging and discharging of condensers 6 and I2 are repeated at a ratedetermined by the frequency of the synchronizing impulses.

Since the time-constant circuits 6, 8, I2 and I3 are identical, andsince they are simultaneously charged and discharged by tube 3, a. trainof saw-tooth voltage waves is impressed on each of the conductors l0 andI4, these waves being of exactly'the same duration, and of exactly thesame amplitude but opposite in phase polarity. Hence this inventionprovides a circuit particularly' well adapted for generating signalsiden- Such signals are ideal as input signals in push-pull or balancedmodulator circuits.

Itis not desired that this invention shall be limited to the exactcircuit elements illustrated in the drawing or to use with synchronizingimpulses of the exact form and frequency illustrated in the drawing. As.this circuit is useful, as

pointedv out heretofore, in any connection where a pair of equal-andopposed saw-tooth voltage waves are desirable, the values of theresistors and condensers have been noted, by way of example only, forthe purpose of illustrating a workable. form of the invention. It willbe obvious. to those skilled in the art that these values are determinedby the frequency and amplitude of the waves which may be required in theparticular Work circuit in which the invention is used.

2,345,668 invention, it will be obvious to those skilled in the art thatvarious changes and modifications may be made therein without departingfrom the invention, and it is, therefore, aimed in the appended claimsto c ver all such changes and modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. A circuit for producing equal and opposite saw-tooth voltagescomprising a thermionic discharge tube, a time-constant circuitincluding resistance and capacitance elements connected to the anodecircuit of said discharge tube, a second time-constant circuit includingresistance and capacitance elements having the same constants as thesaid first time-constant circuit and connected in the cathode ciruit ofsaid discharge tube, a source ofsynchronizing impulses connected to thecontrol electrode of said tube for periodically rendering it conductiveand means for energizing the anode and cathode circuit of said tubewhereby when said tube is conductive the anode circuit of said dischargetube for forming a voltage wave of substantially linearly increasingamplitude, a second identical storage circuit connected in the cathodecircuit of said discharge tube for simultaneously forming a voltage waveof substantially linearly decreasing amplitude, a source ofsynchronizing impulses connected to the control electrode of saiddischarge tube for periodically' rendering said tube conductive andmeans for energizing the anodeand cathode circuit of said tube wherebyto decrease the potential across said first storage circuit and tosimultaneously increasethe potential across said second storage circuitwhen said tube is conductive.

3. A circuit for producing equal and opposite saw-tooth voltagescomprising a thermionic discharge tube, means coupled with said tube forestablishing a predetermined voltage, means con-' nected in the anodecircuit of said discharge tube for forming a voltage wave increasingfrom said voltage, a second means connected in the cathode circuit ofsaid discharge tube for forming a voltage wave decreasing from saidvoltage, a source of synchronizing impulses connected to the controlelectrode of said discharge tube for periodically rendering said tubeconductive and means for energizing the anode and cathode circuit ofsaid tube whereby said first voltage is sharply decreased to saidpredetermined voltage and said second voltage is sharply increased tosaid predetermined voltage when said tube is conductive.

4. A method of producing equal and opposite saw-tooth voltagescomprising the steps of charging a first storage device over a certainperiod of time, discharging a second storage device over the same'periodof timeand utilizing a synchronizing impulse to simultaneously andsubstantially instantaneously discharge said first storage device andcharge said second storage device whereby the voltages across saidstorage devices are substantially equal in magnitude and opposite inpolarity.

CLYDE E. HALLMARK.

